1. Field of the Invention
This invention relates to a method for obtaining a high purity cellulose product from corn hulls. More particularly, this invention relates to the treatment of cellulose products derived from corn hulls and containing hemicellulose to obtain high purity cellulose products and intact hemicellulose or a xylose-containing solution.
2. Description of the Prior Art
Cellulose occurs in nature in the form of fibers which consist of long straight chains of anhydroglucose units. In pulping operations for isolating cellulose from the other constituents of wood or similar materials, some degree of degradation of the cellulose inevitably occurs, leading to a shortening of the average chain length of the cellulose molecules. In general, the longer the average chain length of the cellulose molecules, the stronger will be the fiber in terms of resistance to heat, chemicals, etc. Cellulose fibers whose anhydroglucose chains have not been substantially degraded are referred to as alpha-cellulose.
Hemicellulose is a term which broadly encompasses the principal polysaccharides present in plant material. Included among these polysaccharides are the pentosans such as xylan which may be converted to xylose by acid hydrolysis.
Although small amounts of hemicellulose are desirable in cellulosic products intended for use in papermaking wherein they improve tensile and bursting strength, it is preferred that the hemicellulose be removed to a substantial degree from pulps used for producing rayon and cellulose in order to reduce haze formation.
The majority of chemical grade cellulose products are derived from the wood pulping industry wherein relatively drastic pulping conditions are utilized. Generally, alkaline Kraft cooking or acid sulfite pulping processes are utilized which comprise a delignification, acid hydrolysis, alkaline extraction and bleaching. There are numerous variations of these procedures.
In the acid sulfite pulping process, a major portion of the hemicellulose in the wood is removed during delignification. The pulping liquid is generally highly acidic having a pH of 1 to 2 and usually contains 6 to 8.5 percent SO.sub.2, based upon the dry weight of the material present. In addition, the pulping liquor contains substantial quantities of mono- or di-valent metal ions.
The conditions employed during pulping normally involve heating in the range of 130.degree. to 150.degree. C. at a pressure of 75 to 125 psi for periods of 6 to 12 hours. Additional treatments such as multistage bleaching requiring 3 to 6 steps accompanied by hot extraction with concentrated alkali are utilized to obtain a chemical grade or highly purified cellulose product.
U.S. Pat. No. 3,817,825 to Conca et al. discloses an improved method for obtaining highly purified cellulose. In this method, after bleaching, the cellulose is subjected to a series of chemical treatments to remove the remaining hemicellulose and other impurities. In a variation of the Kraft process, an acid prehydrolysis is utilized before the alkaline delignification step which facilitates the removal of the hemicellulose. Gutierrez Espinosa et al., C.A. 84: 91781h (1976), disclose a method for making a chemical grade cellulose from sugar cane bagasse by a variation of the Kraft process. A prehydrolysis step is required to reduce the hemicellulose content of the cellulose fraction in order to make the latter satisfactory for preparing a viscose solution for the manufacture of rayon fiber. Other methods disclosed in the art for preparing purified cellulose are set forth in U.S. Pat. No. 3,937,647 to Backstrom et al. and Japanese Pat. No. 34,522/73 to Fujioka et al.
There are a number of procedures disclosed in the art directed to the extraction of hemicellulose from fibrous waste products such as corn hulls, cottonseed hulls, peanut hulls, oat hulls and the like utilizing aqueous alkali. Exemplary of such procedures are those disclosed in U.S. Pat. No. 1,819,233 to Darling; U.S. Pat. No. 2,218,567 to White; U.S. Pat. No. 2,709,699 to Wolf et al; U.S. Pat. No. 2,810,955 to Rutenberg et al.; U.S. Pat. No. 2,868,778 to Watson et al.; and U.S. Pat. No. 3,879,373 to Gerrish et al. These procedures generally suffer the disadvantage of producing a colored, impure, hemicellulose product. German Offenlegungsschriften Nos. 2,358,472 and 2,365,457 disclose processes for treating oat hulls to recover xylose, cellulose and lignin.
U.S. Pat. No. 3,716,526 to Schweiger teaches a method for producing a relatively pure hemicellulose product whereby corn hulls are first subjected to an alkaline extraction procedure to produce a crude hemicellulose product and then treating such product with an aqueous organic acid-containing liquid to solubilize the impurities. U.S. Pat. No. 3,970,712 to Friese and U.S. Pat. No. 3,990,904 to Friese et al. disclose methods for treating oat husks to obtain a xylose solution. U.S. Pat. No. 4,038,481 to Antrim et al. discloses a method for treating corn hulls to obtain three fractions therefrom, namely, a cellulosic fraction, which may contain up to 35 percent by weight of hemicellulose, a hemicellulose fraction and a non-carbohydrate fraction. U.S. Pat. No. 4,104,463 also to Antrim et al. is directed to a process for treating corn hulls to obtain therefrom a holocellulose fraction and a non-carbohydrate fraction.